1975 was supposed to be year 6000 of the 7th creative day. So the 1000 year reign HAD to start then to maintain the time table.
I Think a lot of it is habit, along with social networks.
Right now 1914 generation is "explained "word for word from the WT but they start on 3rd base with erroneous assumptions about 607bc-- remember the public WT that said maybe the Bible record missed dates of kings so you cannot count back and get to 607.